1. Field of the Invention
The present invention relates in general to a preparation method of a polyester resin, more particularly, to a preparation method of a polyester resin particles for dry toner preparation used in printers.
2. Description of the Related Art
Toners for laser beam printers are largely classified into two groups: dry toners and liquid toners. Dry toners usually contain binder resins, colorants and other additives.
Among them, the binder resin comprises approximately 90 wt. % of the total weight of the toner, and is responsible for fixing toner particles onto a printing paper. Therefore, the binder resin is the key ingredient having the biggest influence on the performance of the toner. Depending on the preparation method of the toner, different kinds of binder resins are used.
The colorant provides a color to the toner. Colorants are secondary processed products that are prepared by adding a vehicle, a resin and a stabilizing agent to dyestuff or pigments in general. Dyestuff is a coloring matter having an affinity for a fiber, namely, dyeability, and generally contains aromatic rings. The pigment is a coloring powder in white or other colors that is insoluble both in water and oil. The pigment imparts color perceivable by the human eye by selectively reflecting or transmitting visible rays with the chemical structure or particles. Although the pigment has extremely fine particles, unlike the dyestuff, it is insoluble in many solvents, and therefore requires a vehicle. In general, the colorant used in the toner preparation is a pigment.
Colorants provide different colors such as carbon black, and other colors, such as blue, brown, cyan, green, purple, magenta, red, yellow and mixed colors thereof. Examples of pigments include anthraquinone, phthalocyanine blue, phthalocyanine green, diazos, monoazos, pyranthrone, perylene, quinacridone, and indigo pigments.
Besides the above-described ingredients, the toner may contain other additives for improving physical properties. For instance, one of the additives in the toner is a releasing agent, which gives a quick release between a roller and a toner when a toner image is transferred and fixed onto a recording medium, to prevent a toner offset. Many times, the recording medium adheres to the roller due to the toner, so that the recording medium is readily caught in the middle. However, with the releasing agent, this problem may be easily solved and prevented. Wax is generally used for the releasing agent.
There are diverse methods for preparing a dry toner. As far as the toner is concerned, characteristics of the toner particles, such as shape and size, are very important since they are very closely related to the resolution of a final print image. To get a high resolution image, toner particles must be spherical and as uniform as possible. Therefore, there is considerable interest in developing a preparation method for more spherical, finer, and a more uniform range size distribution of toner particles.
In general, the preparation methods of a dry toner are classified into pulverization, polymerization and other chemical methods. According to the pulverization (or milling) method, a binder resin, a colorant, a charge control agent and other additives are preliminarily mixed, uniformly dispersed, and pulverized again.
In consideration of the aforementioned requirements of toner particle characteristics, the toner produced by pulverization has several problems, such as great variations of particle size and shape, and poor yield from the final pulverization process. Therefore, it is difficult to obtain uniformly spherical fine toner articles.
Taking the above shortcomings of the pulverization method into account, a polymerization method would be preferable in preparation of toner particles. According to the polymerization method, raw materials for toner preparation are mixed and polymerized. Examples of the polymerization method include suspension polymerization and emulsion polymerization.
The suspension polymerization is a method wherein water-insoluble monomers are converted into about 10 μm-diameter oil droplets and dispersed in water for polymerization. The method uses a lipophilic polymerization initiator and requires a vehicle for stabilizing the oil droplets.
The emulsion polymerization is a method wherein oil-soluble monomers are emulsified by utilizing an emulsifying agent, and polymerization is initiated with a water-soluble initiator. An ‘emulsifying agent’ includes all the substances that make two non-mixable liquids into a stable emulsion, such as a surfactant which emulsifies water and a water-insoluble organic matter together. A surfactant is an additive that easily adsorbs to the surface and forms micelles when exceeding a critical micelle concentration.
As a rule, the emulsion polymerization takes place in the micelles containing monomer, resulting in polymers of a high degree of polymerization. A micelle is formed as the molecules or ions of surfactants in the aqueous phase aggregate when they reach the critical micelle concentration. In the aqueous phase, a polymerization initiator is radicalized, and a monomer bonded to the radicalized initiator is trapped in the micelle for polymerization. Since the polymerization of monomers takes place within the micelle, the emulsion polymerization method is also applicable to synthesis of submicroscopic micro gel (tens of nm in diameter).
When a toner is prepared by emulsion polymerization, latex is usually used as the binder resin. Latex is the milky white fluid contained in the tissue beneath the bark of the Para rubber tree or Hevea brasiliensis. Rubber particles are dispersed in water (the dispersion medium) forming a colloid phase. Latex is used as a generic name for natural rubber latex, synthetic rubber, and synthetic resin emulsions of a non-rubber group. Examples of monomers used in production of latex are styrene, divinyl benzene, n-butyl acrylate, methacrylate and acrylic acid.
Toner preparation based on the emulsion polymerization method using latex as the binder resin is disclosed in U.S. Pat. No. 6,120,967. According to the disclosure, a monomer selected from a group consisting of styrene, butyl acrylate, and acrylic acid is mixed with an anionic surfactant and an initiator, and the mixture undergoes a polymerization reaction at a predetermined polymerization temperature to produce latex, the binder resin. The produced latex is then mixed with a colorant and a wax that is used as a releasing agent. Later, a coagulant is added to the emulsion for agglomeration, and the resulting agglomerated particles are melted to produce a toner.
As mentioned earlier, compared to other preparation methods, the emulsion polymerization method using latex is more useful for producing fine and uniform spherical particles. Although there is a variety of monomers that may be readily used or commercially available for the emulsion polymerization, styrene/acrylate latex is used most frequently.
Styrene is a general purpose material used in the chemical engineering of resins, synthetic rubbers and paints. Acrylic acid is an easy-to-polymerize material obtained by the direct oxidation of propylene or hydrolysis of acrylonitrile with sulfuric acid. Therefore, styrene and acrylic acid (methacrylic acid) are often used in the production of latex products. To use styrene/acrylate(methacrylate) latex resin for the toner, however, high-level physical properties in thermal or mechanical aspects are required. Also, the low-transparency of the styrene/acrylate(methacrylate) latex resin may present a problem for expressing a color of the toner. Developed later as an answer to the problem is a polyester resin.
U.S. Pat. No. 6,203,957 disclosed a toner preparation method using a polyester resin as a binder resin. According to the disclosure, monomers were polymerized to produce a self-dispersive polyester resin in water. The polyester resin was then dissolved in an organic solvent and mixed with aqueous ammonia as a neutralizing agent. The mixture was dropped into a aqueous medium containing acid to form particles. The resulting particles were filtered, dried, and mixed with a colorant and other additive(s) to produce toner particles.
It is a known fact that polyester resin has superior thermal and mechanical physical properties and excellent color expressive power compared to the existing styrene/acrylate latex. However, the preparation method of polyester resin is somewhat questionable. For instance, U.S. Pat. No. 6,203,957 suggested that a polyester resin should be dissolved in an organic solvent that dissolves the polyester resin, and then dispersed in an aqueous medium. In effect, this is the basis of the production of polyester resin for use in a toner. A frequently used organic solvent for polyester resin is tetrahydrofuran (THF), which is a hazardous substance causing severe damage to the body of a user and environment contamination problems.
In addition, when a toner is produced using the conventional polyester binder resin, it is very difficult to produce fine particles with a diameter of less than 1 μm from the dispersion. Thus, the aforementioned emulsion polymerization method becomes ineffective.
Therefore, there exists a need to develop a new preparation method of a dry toner using a polyester resin as a binder resin, in which the dissolution step in an organic solvent (that is not environmentally friendly) is removed, and emulsion polymerization is used.